Field of the Invention
This invention relates generally to Buck converters, Boost converters, and other types of switching converters, and the use of a current sensor to sense and control the current in the switching converter.
Description of Related Art
A current sensor may be used in a switching converter to measure input or output current and to generate a signal proportional to it. The generated signal may be an analog or digital current and utilized as a current source or for current limiting.
FIG. 1 shows a typical circuit topology 100 of low side current sensor 105 and corresponding output stage 110 of a switching converter. Low side current sensor 105 is comprised of a standard op-amp with negative feedback, constant bias current IREF and NMOS device 130, configured to non-inverting input (V+), and NMOS sense device 140, configured to inverting input (V−). The supply voltage for bias current IREF limits the op-amp's output voltage, which in turn limits sense current ISENSE. Output stage 110 is comprised of PMOS high side device M1 and NMOS low side device M2, driven by PMOS and NMOS drivers respectively. The drains of devices M1 and M2 are connected at voltage VLX, and to inductor L, which drives the load current IL into capacitance C and resistance R.
The amount of sense current will parallel that of the load current and an averaged sense current may be required. Filtering may be used to obtain the sense current, limiting the frequency bandwidth of the current sensor. Since VLX and IL of FIG. 1 contain a wide range of frequency components, op-amp 120 needs a wide bandwidth to follow VLX and IL, and any filtering will degrade the sense current accuracy.
FIG. 2 illustrates waveforms 200 of a typical low side current sensor and corresponding output stage of the switching converter in FIG. 1. PMOS high side device M1 is on when VLX is high and load current IL rises. NMOS low side device M2 is on when VLX is low and load current IL falls. Sense current ISENSE turns on when VLX goes low and falls from positive to negative in value. Positive current means current flows out of NMOS device M2 and negative current means current flow into NMOS device M2.
FIG. 3 shows a derivative current sensor employing a low pass RC filter, as a pre-filter to op-amp 305. PMOS high side device M4 and NMOS low side device M5, are driven by PMOS and NMOS drivers, respectively. Current sensor 300 includes on resistance RS, measured across the output of sense devices M4 and M5. On resistance RS of high side device M4 determines voltage VP, receives feedback current IFB set by current mirror devices M1, M2, and M3, and is an input to switch 310. On resistance RS of low side device M5 receives bias current IREF, which determines voltage VREF, and is an input to switch 315. Sense current ISENSE includes high side sense device current when input switch 310 is closed and switch 315 is open, and low side sense device current when input switch 310 is open and switch 315 is closed. Switches 310 and 315, when closed, configure on resistance RS to low pass filter resistance RLPF, and inverting input (V−) and non-inverting input (V+) and of op-amp 305, respectively. The output of op-amp 305 determines voltage VO and is the input to load resistance RO and load capacitance CC. Capacitance CLPF, along with resistance RLPF, comprise the low pass RC filter.
The current sensor of FIG. 3 generates VREF from a given IREF, and on resistance RS of the sense device, to determine a static operating point. In addition, a certain amount of current is generated by VREF and on resistance RS, even when the load current is not being generated. During this time, VP is maintained at exactly the same voltage as VREF, through negative feedback of the current mirror devices. This creates a certain offset current IOC, which combines with output sense current ISENSE. Because reference voltage VREF has some amount of error due to process variation, temperature, and supply voltage, offset current IOC further degrades the accuracy of output sense current ISENSE.
In addition, the current sensors shown in FIGS. 1 and 3 may detect only positive current because the current sensor does not sink sense current. A small amount of negative current may be detected by increasing the offset current, but this is a natural limitation for these types of current sensor topologies.